The invention relates to a voltage/frequency converter with a first change-over switch which supplies in its first position a measurement voltage and in its second position a reference voltage to a first input of a comparison circuit, and a second change-over switch which supplies in its first position a first signal to be integrated and in its second position a second signal to be integrated via an integrator to the second input of the comparison circuit which generates pulses dependent on the measurement voltage and which sets the two change-over switches into their first position when the voltage supplied by the integrator is equal to the reference voltage and into their second position when the value supplied by the integrator is equal to the measurement voltage.
Such a voltage/frequency converter converts an analog voltage into rectangular oscillations (rectangular pulses) the frequency of which is inversely proportional to the amplitude of the voltage applied. A rectangular pulse exhibiting a constant amplitude is followed by a pulse gap. The duration of the pulse and the pulse gap is defined as the duration of the period which corresponds to the reciprocal value of the frequency.
The initially mentioned voltage frequency converter is known from DE-AS No. 2,135 802. In this arrangement, a measurement voltage or a reference voltage is supplied by a firstchange-over switch to a first input of a comparison circuit. An output of an integrator is connected to the second input of the comparison circuit. The input of the integrator is supplied with a first or a second voltage to be integrated from a second change-over switch. The two voltages to be integrated exhibit an inverse polarity. In the first position of the first change-over switch, the measurement voltage is switched to the first input of the comparison circuit and in the first position of the second change-over switch the first voltage to be integrated is switched to the integrator. The comparison circuit generates a positive pulse if the integrated voltage becomes equal to the measurement voltage. This causes the two change-over switches to be switched over into their second position. In the second position of the first change-over switch, the reference voltage, which is smaller than the measurement voltage, is present at the first input of the comparison circuit and in the second position of the second change-over switch the second voltage to be integrated is present at the integrator. If the output voltage of the integrator reaches the level of the reference voltage, the pulse generated by the comparison circuit is terminated and the change-over switches are reset to their first position.
The duration of the period of the pulses generated by such a voltage/frequency converter approaches zero if the reference voltage is equal to the earth potential and the measurement voltage becomes smaller and smaller. This means that the duration of the period has a very great range of variation and there is no possibility of determining the duration of the time between two rectangular pulses if the measurement voltage is very low.